there is a large stud farm and riding centre called Kabiuk and it is as you come from Varna on main Shumen motorway and keep heading for Shumen till you are at last junction from the motorway for turning into Shumen...don't turn right there go to next right turn (if you see a roadside red restaurant on your left you have gone too far)and continue up the road it is sign posted on left, just keep driving up till you see it.
They have an arena there and stables and possibly do hacks out.
It's a pity you were not nearer we have some great stables in our village And some beautiful horses too might be worth a day trip so i have let you know anyway